1. Field of the Invention
The present invention relates to an image forming apparatus, in which an image is formed by developing an electrostatic image formed on an image bearing member with a developer.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic system, a developing device for developing an electrostatic image formed on an image bearing member has been conventionally exemplified by a developing device using a two-component developer including non-magnetic toner particles (i.e., a toner) and magnetic carrier particles (i.e., a carrier). Such a two-component developer need not include a magnetic material in a toner, and therefore, has been widely used in, particularly, a color image forming apparatus for reasons of a good tint and the like.
In the above-described developing device for use in the image forming apparatus, a developer contained inside of a development container is agitated and conveyed to a developing sleeve 100 serving as a developer bearing member by an agitating/conveying member, as illustrated in FIG. 6. The developing sleeve 100 incorporates a magnet 101 therein. The conveyed developer is borne on and conveyed by the rotating developing sleeve 100. And then, the developer is supplied to a photosensitive drum serving as an image bearing member in a state in which the thickness thereof is restricted by a developing blade 102 serving as a developer restricting member, thereby developing the electrostatic image formed on the photosensitive drum.
In the above-described developing device, any foreign matter has accidentally clogged at a position at which the developer borne on and conveyed by the developing sleeve 100 is restricted by the developing blade, thereby inhibiting formation of a layer of the developer. A quantity of the developer becomes smaller at that position of the inhibition, thereby raising a problem that fundamental development cannot be carried out.
In order to solve the problem, there has been proposed a method for fixing a member for vibrating the developing blade per se so as to remove foreign matters clogging in a gap defined between the developing blade and the developing sleeve (see Japanese Patent Application Laid-open No. 11-231645).
Incidentally, in the developing device for restricting the thickness of the layer by the developing blade 102, as described above, a portion A, at which a flow of the developer borne on and conveyed by the developing sleeve 100 is dammed by the developing blade 102, is generated at a developer sump 103. In contrast, the developer borne in the vicinity of the surface of the developing sleeve 100 is conveyed at substantially the same speed following the rotational speed of the developing sleeve 100. As a consequence, a shear surface is produced on the developer on a boundary between the dam portion A and a conveyance portion B, and then, a toner is liberated due to a difference in flow at the shear surface, resulting in formation of the layer of the developer at that portion. When the layer of the developer grows, the gap defined between the developing blade 102 and the developing sleeve 100 becomes smaller. The thickness of the layer of the developer formed on the developing sleeve 100 at a portion having the smaller gap becomes smaller than those at other portions. As a result, the developer provided for the development becomes smaller in quantity, thereby making an image pale. A study by the Inventors has found that this phenomenon is liable to occur in the case of the use of the two-component developer incorporating the toner and the carrier therein.
Furthermore, it has been found that the above-described phenomenon tends to cause the developer to be hardly consumed in the case of, in particular, a low printing ratio, to increase an aggregation degree of the developer due to liberation or embedding of an additive agent added into the developer, and to lead to the growth and marked generation of the layer of the developer.
In view of this, it is construed that the growth of the layer of the developer is suppressed by adopting a configuration in which the developing blade is vibrated, as disclosed in JP-A No. 11-231645. However, if the developing blade is vibrated in the case where an image is formed in a high printing ratio, the developer tends to fly more due to the vibrations. Moreover, the repeated vibrations induce contamination inside of a main body with the flying developer or adhesion of the flying developer to a recording sheet, thereby possibly degrading the quality of the image.